Telephone repeater



April 9, 1935. I w. H. MARTIN 1,997,223

TELEPHONE REPEATER Filed Oct. 28, 1955 8 g .D *3 2% J 3 $2, c 6 X 4 Current .Hin'vzlny fi om East Zine INVENTOR WEMZU ZLM/ BY W ATTORNEY I Patented Apr. 9, 1935 TELEPHONE REPEATER- 5 William Hennick Martin, Chappaqua, N."Y., as-

signor toAmerican Telephone and Telegraph Company, acorporation of New York- AppIication October ZS, 1933, Serial No 695,878

I s Claims. (01. 179-170) This invention relates to two-wire telephone repeaters, and more specifically to repeaters of the so-called directional type in which an attenuation pad is transferred from one side of the repeater to the other'by a voice-operated relay,

in accordance with the direction of arrival of speech currents. Itis an object of the invention to limit the circulating currents through the repeater dueto mpedance unbalance between the: lines and their respective networks, thus permitting the repeaterto give a larger gain than would otherwise be possible under the line balance conditionsobtainable. A further. object isto improve the operation of the voice-operated system by increasing the operating differentialswithoutincreasing the losses; to speech. A third object 'is to improve the discrimination ofthe device against false operation due to noise.- Another object is to'decrease the-loss suffered by voice currents which are too weak to afiect the voiceoperated mechanism; v r a The principle of the directional repeater is describedin a patent application} Serial Number 687,880, filed by L. G. Abraham and. D. Mitchell on September 1, 1933, and another application, Serial Number 687,874,- filed by- B.- -F. Lewis and J. L. Merrill, Jr.', on September 1, 1933,=both of which applications are assigned to the sameassignee as the present one.- Brieflystated'sucha repeater is of; the ordinary 22-type, with theadditionof a voice-operatedsystem which makes an-.increase;in1 the gain of one side of the repeater, and;a corresponding decrease-in thegain of the. oppositeside,v in. accordance with the. direction of. arrival of voice currents at the repeater. A net-loss can thus be maintained in the path of circulating currents within the repeater, while a'net gain is provided in the direction in which speech currents are being transmitted. Again can therefore be obtained with lines'whose impedance characteristics are soirregular that a suitableimpedance balance cannot be obtained by-any ordinary means. The arrangement is also useful where the repeater may be switched to any one of several lines having different impedance characteristics.

The'present invention'consists, ingeneral, in theaddition to one. ormore ofthe-atte'nuation pads which are controlled by'the voice-operated system, of-a unit or units-having a variable-resistance characteristic, suchas a copper-oxide rectifier, for example. The method'ofapplication and.theadvantages of' such an arrangement will appear from. the following detailed description, which is to be read in connectionthe Figure l of the .f,.drawi ng I attached drawing. shows schematically a circuit comprising,,;one embodiment of the invention. Fig. 2'illustra'tes graphically someof the advantages of the pro posed system,

In Fig. -1 of the drawing, the heavylinesshow the essential elements of a conventional 22-.type

repeaten'compri'sing the east and west networks,

intended to balance the eastJandwest'lines-reaspectively, the hybrid coils'A and A, andIthe amplifiers B and B, interconnected by. the conductors D',E, and.D,;E'. Such a. repeater may be converted into the directional type by the addition of thearnplifier-rectifiersC andC, differentially connected to the polarized relay F,

which controls the subsidiary rela'ys liandl'. These relays in turn control the? padsLJ,v K. and J K. on the arrival. of speech. currents from the west lineff or example, aportion of these;cur rents is amplified-and rectified through C, and iiowsfthrough winding.,G.of .relayl .v This wind.- ing is so poledas to'm'ake the armature of relay F take theposition shown, against'th'e left-hand contact; Current thereupon flows through lbiasingwindingH, which is so-poled as to hold relay F that position, and I through the winding, of relay 1, operating 'thelatter; as indicated in the drawing. Pad K is thus disconnected trombonductors Efand pad J isconnected across conductors D. Meantime, since relay. If is not operated, pad- K" isconnectedthro-ugh the bacl; contact of relay I to conductors E. :Thus pads J and K .are inserted in..the lower half of therepeater, whilefpadsJ. and K areremovedirom hand contact, even after-voice currents have ceased to arrive-from the west line. Whenthe subscriber at the east end of the circuit begins to speak, howev'er, rectified current from amplifier-rectifier C'. willflow: through windingr Gt, whichis opposed to windings G and H. When the current in winding G'becomes great enough to overcome the biasing-flux due to thecurrent in winding 11,- thearmatureof relay F will be,

voltage characteristic.

7 will send current through biasing winding H,

holding the armature in this position, while relay I will be released and relay 1 operated. Pads J and K will thus be removed from thelower half of the repeater, while pads J andK will be inserted in the upper half. Gain will be provided for speech currents arriving'from the east line, and objectionable circulating currents will be prevented by pads J and K. Relay F will reverse each time the direction of speech changes, so that the gain of the repeater will always be, provided in the proper direction. This is the principle of the directional repeater, as .covered in the patent applications already referred to.

r In accordance with the present invention, I propose to include in some or all of the pads, resistance units having a" variable resistance- These might conveniently take the form of copper oxide rectifiers. A pair of such rectifiers, oppositely poled to provide full- Wave operation, is indicated in the drawing as included in each pad. For example, the pad K is made up of the fixed resistance L, shunted bythe rectifiers M and N; oppositely poled. Polarizing batteries P and Q are shown: these may or may not be desirable. The whole combination is connected across conductors E through the transformer R; Pad L is similarly made up of fixed resistance L, rectifiers M and N, and batteries P and Q, all connected across the line through transformer R. Pad J .is made up of a fixed resistance, inductanceand capacity multiple, and in parallel thev rectifiers S and-T,jconnected through transformer U: no polarizing batteryis shown in this case. Pad J is similar to pad J.

The general characteristic of a'padsuch as has been described is'thattheloss which it introduces will vary with the intensity of the speech currents flowing in thecircuit, the loss being greater for large currents than for small. This is an undesirable characteristiefor'a pad ina talking circuit, since it will tend'to. cause distortion by attenuating the peaks of current, such as those corresponding to vowel sounds, more than the smaller currents. It is particularly'suitable to the directional repeater, however, since the pads are not in the side of the repeater which is transmitting speech, but in the opposite side, where distortion is of no consequence. Any usefulspeech currents which would pass through the pads would be so weak as to be practically unaffected, since any such currents that were intense enough to be appreciably affected by thepads would be sufiicient to operate the relays, and remove the pads from thatside of the repeater.

, There are, moreover, numerous substantial advantages in the use of this type of pad in a directional repeater. In the first place, an extra degreeof protection against singing will .be provided, since the building up of a circulating current will be automatically checked by the increase in the loss of the pads with increase in current. Violent singing will be avoided altogether. Furthermore, some of the most objectionable effects of a near'approach to the singing condition will be minimized. .As is well known, when a 22-typ'e repeater is slightly below the point of sustained singing, serious distortion is experienced due to the fact that the louder sounds of speech are accompanied by a metallic ringing sound. This is caused by circulating currents within the repeater, set upby the peaks of voice of time becauseof the insufiicient damping in the circulating path. With the type of pad contemplated in the present invention, such large currents would encounter an increased loss, and the ringing sound would be much reduced. It would thus be possible to operate the repeater nearer to the point of actual singing without objectionable distortion. A useful degree of protectipn would be afforded against singing or the adverse efiects of a near approach to singing.

. In addition to furnishing this protection, the use of this type of pad will make a substantial improvement in the operation of the directional repeater. Since the relay F is differentially wound, the efiective flux tending to operate it or to hold it operated depends upon the difference between the currents in the windings H and H. On account of the poor balances that will generally exist between the lines and their respective networks, there will be considerable transmission through the hybrid coils; that is, speech currents arrivingfrom the west line will reach amplifier-rectifier C as well as C, and vice-versa. Unless the loss through the hybrid coils is greater than the gain in the repeater amplifiers, false v rents fromthe east line will never be able to operate relay F in the reverse position. Correspending relations will of course be necessary.

with respect to the pads J and K when the system is operated in the other direction. 3

Consider specifically'the conditions necessary to permit currents from the east line to operate the relay F when it is in the position shown in the drawing. With a poor balance between the west line and its network, the loss inpad K will have to be comparatively large to permit operation. With a fixed pad, this will mean a comparatively large loss to voice currents which are too weak to operate the device, so that initial consonant sounds of low intensity will be greatly attenuated. By using'a pad constructed in accordance with this invention, such weak currents will be less attenuated, since the loss of the pad will be less for weak currents than for strong currents. This difference is illustrated in Fig. 2 ofthe drawing. Line A represents the flux in winding H, increasing in direct proportion to the magnitude of the incoming current. Line B represents the flux in' winding H, opposing the operation of the relay, if an ordinary fixed pad is used for K This flux also increases in direct proportion to the incoming current. The difference between line A and line B is the net flux tending to operate the relay: the length of the line D represents the fluxnecessary for operation. Line B also indicates, to an appropriate scale, the current transmitted to the west line. Line ,0 similarly indicates the flux in winding H, and the current transmitted to the west line, when the pad K is designed in accordance with the present invention,the proportions being such as to produce the flux necessary for operation at the same value of input current. -Line C lies above line B up to the point of intersection X, since the loss of pad K is less, for these low in 'tensities, when designed in accordance with this invention. 7 Y Hence the magnitude oi? the low 'intensity currents transmittedto thewest line is substantially greater with typeot pad tlian was reached; Speechcurrents, however, are very' irregular, with frequent peaks'ofshortduration, several times as greatas the average intensity of" the current. -It will be seen 'that' the situation with the new*'type-ofpad, indicated by line C, is considerably more favorable to the operation; of the-device'- on such short peaks than with the fixed pad, since the flux differential, represented by the difierence be tween lines A and C, is considerably greater than the corresponding difference between, lines A and B for any point to-the right of line D. The sensitivity ofthe device tospeech currents; therefore, is greater with thenew type'ofpadthan with the usual type,'ev"en when the nominal sensitivity 'is adjusted to-be'the same, based on measure.-

inents made with steady single-frequency currents, as is the usual practice. The sensitivity'to operation from-noise currents, however, willimore nearly approximate the nominal sensitivity, since in general noise currents are more steady in character than speech currents, witha smaller percentage of relatively high peaks. The effective sensitivity to speech currents will thus be'substantially higher than that to noise currents,- a very desirable characteristic, since false operation due to noise usually represents the practical limitation on the sensitivity that can be used.

As has already been stated, it may or may not bedesirable to use polarizing batteries, such as P, Q, P, Q, in connection with the rectifier units. Isolated batteries of this kind are in general objectionable, if their use can be avoided; The purpose of providing them would be to have a means for adjusting the magnitude of the losses introduced by the pads. The same result could be obtained to some degree by properly selecting the transformation ratiosof the transformers R and R, and the magnitudes of the resistances L and L. If the transformers are arranged to step up the voltages applied to them, the losses caused by the pads will increase more rapidly,

with current.

Up to this point no reference has been made to the effect of including rectifier units in the input pads J and J. In many cases it will probably be undesirable to use this type of pad in these locations, since they will have a tendency to re- .duce the operating sensitivity of the device,

though they will have no eifect on the differential action of the pads K, K already explained. They may be used in some cases to improve the holding difierential on the louder portions of the speech currents, if the loss of. the fixed pad is not quite great enough, together with the loss across the' 1 echo currents to small. values.

stantially equivalent to a'22-type repeatenas in:-

dicated by the'h'eavylines in Fig. 1 of the" draw-'- ing, except-that the two hybrid sets may-be 10 cated many. miles apart, and "several .one-way amplifiers may be" inserted at intervals. in the I four-wire lines D, Eand D-"E ents 'throughthe two sides'of the four-wire ciredit in tandem, due to unbalancesat the hybrid Circulatingcursets, are objectionable in this case, as they arein the ZZ-t'ype repeater.'"They may cause singing of the -four-wire systemas a whole, if the terminalunbalances are severe and'the'intennediate gains arehigh. Morecommonly the magnitudes of the circulating: currents are less than this,.but

they may become objeotionable.because of delay in transm'ission over the long four-wire lines. Under these circumstances they produce 'thezeffeet" of distinct echoes fof speech' in the subscribersi receivers, and are hence generally known as echo currents. Voice operated'devices known as echo suppressors, similar in principle to the directional device herein described, are often: used on, such long four-wire circuits, for the purpose of: introducingattenuation in thereturn path, andathereby reducing these'circulating or It is obvious'irom the foregoing discussionthat advantages maybe ioun'dfin theuse in such echosuppressors of pads designed in accordance with thepresentinvention, since such'pads willcause a loss to the'echo currents increasing with the. intensity of. those currents, while causing a comparatively 'small' lossito idirect" transmission of insufiicieht intensity to operate the echosuppressorr r It should be noted that the form of the variable resistance units is not limited to copper-oxide rectifiers, although the discussion has for convenience been mainly given with reference to 7 considered as limited to the shunt arrangements shown, but may take a variety of forms which will be obvious to those skilled in the art. In general, while the invention has been disclosed in certain specific embodiments for purposes of illus tration, it will be understood that it may be embodied in other and difierent forms without departing from the spirit of the invention, which is defined in the appended claims.

What is claimed is:

1. In a two-way signaling system, the method of obtaining amplification of. the signals which consists in providing separate paths for the two directions of transmission, providing unidirectional amplification in each of the paths, making a continuous electrical comparison between the strengths of the signals in the two paths'and.

introducing in the path having the lower signal strength an attentuation varying with the strength of the signal voltage in said path.

2. In a two-way telephone repeater, the method of avoiding objectionable circulating currents within the repeater due to unbalances between the connected lines and their respective networks,

, in said path.

3. In a two-way signaling system, two paths for transmission of signals in the two directions, a unidirectional amplifier in each of said paths, an attentuation element adaptedto be inserted in one of said' paths, electrical means for comparing the strengths of the signals in the two paths, means operated by the signals for inserting said attentuation element in the path in which strength of the signal voltage applied to said element.

4.;In a telephone system having two unidirectional paths for transmission in the two directions, an attenuation element, voice-operated means for inserting said attentuation element in one path or the other in accordance with the direction in which telephonic currents are being transmitted, and means associated with the attenuation element for giving said element a loss characteristic varying with the strength "of the telephonic voltage applied to'said element from the path in which it is inserted.

5. In a two-way telephone repeater, a plurality of attenuation elements, voice-operated means for transferring one or more of said attentuation elements from one side, of the. repeater to the other in accordance with the direction of; arrival of speech'currentsv at the repeater, andmeans included in one ormore of said attenuation elements for giving such element or elements a loss characteristic varying with the strength of the voltage applied thereto from the side of the repeater in which the element is inserted.

6. In a two-way telephone repeater, two paths for transmission in the two directions, a unidirectional amplifier in each of said paths, an attenuation element adapted to be inserted mom of said paths, a voice-operated device difierentially connected to corresponding points in said two paths, means controlled by said voice-operated device forinserting said attenuation element in one or the other-of said paths, and means associated with said attenuation element for giving said elementa loss varying with the strength of the telephonic voltageapplied to said element ,from the pathin which it is inserted.

'7. In a two-way signaling system, two paths for transmission of signals in the two directions, a

unidirectional amplifier in each of said paths, an

attentuation element adapted to be inserted in one of saidpaths: and characterized in that its loss 'varies withthe strength of the signal voltage applied to it from the path in which it is inserted,

electrical means for comparing the strengths of the signals'fin the two paths, and means operated by the signals'for inserting said attenuation element in the path in which the signals are weaker.

and further varying the amplification in one path in accordance with the absolute signal strength in thatpath. I

WILLIA -H. MARTIN. 

